Exercise: Drawing cloud formations

This is as difficult as drawing animals.  You just get set up and draw a few squiggles for the outline, and then look up again only to find the whole thing has moved.  I need to try again on a very still day!

The next exercise was plotting space through composition and structure.  I think in my drawing I have achieved a foreground, middle ground and  background, but I did have difficulty in varying the tone sufficiently to convey the feeling of distance in the landscape.